1. Field of the Invention
The invention relates to a digital duplex communication system for data signals having a power-versus-frequency spectrum comprising two side bands located one each side of a symbol frequency, the system comprising a transmitter/receiver including an echo canceller, the received signal being sampled at a multiple of the bit rate and comprising in each symbol period a first time during which data is valid and a second time during which data is invalid, the data being decoded from the value of the sampled signal during the first time.
2. Description of the Prior Art
Such a system is described in U.S. Pat. No. 436,533, filed 10-25-82 in which the received signal, which is encoded according to the WAL2 code, is sampled at four times the bit rate and the echo canceller produces an echo replica at each sampling instant which replica is subtracted from the received signal. The received signal produces an eye diagram having a true eye which corresponds to the first time during which data is valid and a false eye which corresponds to the second time during which the data is invalid. An ambiguity detector such as that disclosed in U.K. Patent Application No. 2048016A is included in the receiver to detect which is the true eye so that the data may be extracted from the true eye only.
In the system described in the first aforementioned patent application the echo canceller has to produce echo replica signals at four times the bit rate thus increasing the speed at which a transversal filter type canceller has to operate or increasing the storage capacity needed in a look-up table type canceller.